Films of polymeric linear polyesters have excellent draw orientation and have proved especially well suited for biaxial film orientation. These polymeric films, especially those of polyethylene terephthalate (PET), are strong and have excellent inherent chemical and thermal properties. In addition, they have good optical clarity, toughness, and static properties, which makes them extremely suitable for use a myriad of uses.
In order to enhance the commercial value of biaxially oriented polyester film, polymer containing coatings are applied to the film surface to promote adhesion, antistatic, release, water absorbency, oxygen barrier, water barrier, metallization, heat seal, and printing properties. Due to the poor static and dynamic coefficient of the coated surface, the coatings tend to block during the winding and slitting operations, producing poor quality mill rolls and slit rolls.
Submicron particles of alumina, silica, titania, zirconia, organic particles such as polymethyl methacrylate and polystyrene, and organically modified silicon beads have been added to the coating formulation as antiblocking agents, both in off-line and in-line in an interdraw process. When applied off-line, the antiblocking particles stay tightly bound to the coated surface of the polyester. However, when the coating containing the filler particles is applied on-line in an interdraw coating process, the particles tend to void during the transverse, or sideways, draw. The leads to undesirable filler wipe off. Wipe off, which causes the coating to lose its anti-blocking properties, becomes more serious as the size of the filler particles increases.
Coatings are preferably applied in-line at the interdraw stage, that is, after the film has been stretched in the longitudinal direction but before the film has been stretched in the transverse direction. Off-line coating requires an additional step to unwind the uncoated film, coat it, and then rewind the coated film, making off-line coating time consuming and expensive. Thus, a need exists for interdraw coatings for polyester film in which the filler particles are not subject to wipe off.